Hydrothermal Processes as an Alternative to Conventional Sewage

Sludge Management

Christian Israel Aragon Briceño

Submitted in accordance with the requirements for the degree of

Doctor of Philosophy

The University of Leeds

Faculty of Engineering

School of Civil Engineering

October 2018

i

The candidate confirms that the work submitted is his own, except where

work which have formed part of jointly-authored publications have been

included. The contribution of the candidate and the other authors to this work

has been explicitly indicated below. The candidate confirms that appropriate

credit has been given within the thesis where reference has been made to

the work of others.

1. *Aragón C., Ross A. and Camargo-Valero M. (2018). “Evaluation and

comparison of product yields and bio-methane potential in sewage

digestate following hydrothermal treatment.”. Applied Energy, Volume

208, 15 December 2017, Pages 1357-1369 .

https://doi.org/10.1016/j.apenergy.2017.09.019.

2. *Aragón C., Grasham O., Ross A., Dupont V. and Camargo-Valero M.

(2018). “Hydrothermal Carbonization of Sewage Digestate: Influence of

the solid loading on hydrochar and process water characteristics”.

Submitted on the 24th of January 2019.

Publication 1 contributed to Chapter 4 and publication 2, contributed to

Chapter 5.

Contributions to publications were as follows:

Data collection and laboratory analysis: I conducted all the

experimental work and data analysis described in the publications. Mr.

Grasham contributed to computer modelling in ASPEN Plus presented

in Chapter 5.

Writing the paper: I was lead author of all the publications. I defined

the structure of the paper and wrote all the sections, which were

reviewed by my supervisors and other co-authors. Their comments

were added accordingly.

https://doi.org/10.1016/j.apenergy.2017.09.019

ii

This copy has been supplied on the understanding that it is copyright

material and that no quotation from the thesis may be published without

proper acknowledgement.

Assertion of moral rights:

The right of Christian Israel Aragon Briceño to be identified as Author of this

work has been asserted by him in accordance with the Copyright, Designs

and Patents

Act 1988.

© 2018 The University of Leeds and Christian Israel Aragon Briceño

iii

Acknowledgements

I would like to express my deep gratitude to my family who has supported me

throughout this journey called PhD. To my wife (Vicky) and my lovely baby

(Luz).

To the Research of Science and Technology Council of Mexico (CONACYT)

for the financial support given to study my PhD at University of Leeds.

I would like to express my gratitude to Dr. Miller Alonso Camargo-Valero who

accepted me into his research group, his support and belief in independence.

This has made me a better researcher. I would also like to acknowledge my

co-supervisor Dr Andrew Ross (Andy) for his moral and technical support

and all the good chats that we had.

I would like also to give my appreciation to the Staff and friends from the

School of Civil Engineering and Energy building, who helped me to spend a

good time during my PhD (academic and social life), Dr David Elliot and Ms

Sheena Bennett, Dr. Karine, Dr. Simon, Dr. Adrian, Mariana, Cynthia,

Cigdem, Legaire, Dorian, Zaim, Andrea, Suha, Ikpe, Godwin, Kiran, Aaron,

Aidan, Chibi, Habeeb and Shehzad. Also, I would like to thank Yorkshire

Water and particularly to Mr Gavin Baker and Mr. Andrew Bowmaker for their

assistance during sampling at Esholt Wastewater Treatment Works in

Bradford and for providing information regarding the operation of a full-scale

sewage treatment system.

iv

Abstract

Sewage sludge management is one of the biggest concerns to the

wastewater industry due to the increasing volumes produced and new

stringent environmental regulations. Hydrothermal Treatments (HT) are a

good option for converting wet biomass such as sewage sludge into high

value products. However, HT are still not well developed when compared

with other waste processing treatments. One of the most promising areas for

developing hydrothermal processing applications is in sewage sludge

treatment facilities. Sewage sludge has been identified as a potential

feedstock for hydrothermal processing that could make use of existing

facilities currently in place in wastewater treatment works (WWTWs). In order

to look for options aimed at reducing the costs of the WWT process and

digestate management by delivering a sustainable and novel approach, the

aim of this project is to assess alternatives to enhance the way sewage

sludge is handled in WWTWs, by focusing on the use of hydrothermal

processes and the potential of recovering energy and nutrients. The potential

of integrating HT Processes with AD for sewage sludge treatment was

evaluated. Hydrochar yields ranged from 38 to 68% at 160°C and from 29

and 40% at 250°C for all thermal treated sewage sludge samples. The

soluble fraction of organic carbon increased in primary sludge digestate

(525%), secondary sludge digestate (808%) and sewage digestate sludge

(675%) after thermal treatments compared with the untreated digestates.

Figures from Biomethane Potential (BMP) tests showed that hydrothermal

treatment enhanced methane production in all non-AD and AD sludge

samples processed. Mass and energy balances were carried out from six

proposed process configurations from different sewage sludge feedstocks

and their digestates (primary, secondary and 1:1 Mix) in order to evaluate the

waste generation, nutrients potential fate, net energy production and

potential profit. The results showed the HTC at higher temperatures (250°C)

seems to have more economic and environmental benefits. Scenarios that

involved primary and mix sludge seemed to be the most suitable options in

terms of the organic matter removal, energy harnessing and economic

feasibility.

v

Table of Contents

Chapter 1. Introduction .................................................................................. 1

1.1. Background ......................................................................................... 1

1.2. Aim, scope and objectives ................................................................... 5

1.3. Structure of Thesis .............................................................................. 5

Chapter 2. LITERATURE REVIEW ................................................................ 8

2.1. Water Supply in the UK ....................................................................... 8

2.2. European Water Framework Directive (WFD) ..................................... 9

2.3. Sewage Sludge in the UK .................................................................. 11

2.3.1. Sewage Sludge Management ..................................................... 12

2.4. Anaerobic Digestion in UK ................................................................. 13

2.4.1. Anaerobic Digestion Pre-treatments ........................................... 15

2.5. Thermal Hydrolysis ............................................................................ 17

2.6. Hydrothermal processes .................................................................... 19

2.6.1. Hydrothermal Carbonization (HTC) ............................................. 22

2.6.2. Hydrothermal Liquefaction (HTL) ................................................ 23

2.6.3. Hydrothermal Gasification (HTG) ................................................ 24

2.7. Nutrients pollution in wastewater ....................................................... 24

2.7.1. Phosphorus ................................................................................. 25

2.7.2. Nitrogen ...................................................................................... 26

2.7.3. Recovering treatments for phosphorus ....................................... 28

2.7.3.1. Struvite Crystallization .......................................................... 29

2.8. Summary of literature review findings................................................ 30

2.9. Statement of Research Problem and current Research Gaps ........... 31

Chapter 3. Research Methodology and Analytical Methods ........................ 32

3.1. Materials ............................................................................................ 32

vi

3.1.1. Seed Inoculum ............................................................................ 32

3.1.2. Sewage sludge samples ............................................................. 32

3.2. Methods ............................................................................................. 33

3.2.1. Feedstock characterization ......................................................... 33

3.2.2. Hydrothermal treatments ............................................................ 34

3.2.3. Characterization of the liquid products ........................................ 35

3.2.4. Characterization of the solid products ......................................... 35

3.2.5. Biochemical methane potential experimental (BMP) tests .......... 36

3.2.6. Biogas composition ..................................................................... 38

3.3. Data processing and analysis ............................................................ 38

3.3.1. Biochemical Methane Production (BMP) .................................... 38

3.3.2. Theoretical BMP (BMPth) ............................................................ 38

3.3.3. Anaerobic biodegradability (BD) ................................................. 39

3.3.4. Hydrochar Yield .......................................................................... 39

3.3.5. Carbon recovery in solid and liquid fractions after HT processing

.............................................................................................................. 39

3.3.6. High Heating Value ..................................................................... 40

3.3.7. Thermal treatment energy calculations ....................................... 40

3.4. Objective 1: Evaluation and comparison of product yields and bio-

methane potential in sewage digestate following hydrothermal treatment 41

3.5. Objective 2: Hydrothermal Carbonization of Sewage Digestate:

Influence of the solid loading on hydrochar and process water

characteristics. ......................................................................................... 42

3.5.1. AD-HTC model............................................................................ 44

3.6. Objective 3: Evaluation and comparison of product yields and bio-

methane potential from hydrothermally treated sewage sludge. .............. 45

vii

3.7. Objective 4: Mass and Energy Integration Study of Hydrothermal

Processing with Anaerobic Digestion of Sewage Sludge ......................... 46

Chapter 4. Evaluation and comparison of product yields and bio-methane

potential in sewage digestate following hydrothermal treatment .................. 49

4.1. Introduction ........................................................................................ 49

4.2. Results and discussions .................................................................... 52

4.2.1. Chemical Oxygen Demand and nutrient balance of thermal

products ................................................................................................ 52

4.2.2. Hydrochar Characteristics ........................................................... 55

4.2.2.1. Elemental composition in Hydrochar .................................... 56

4.2.2.2. Energy characteristics of Hydrochar ..................................... 57

4.2.2.3. Carbon balance in the Hydrochar ......................................... 59

4.2.3. Characteristics of process waters ............................................... 60

4.2.4. Anaerobic digestion of HT Slurries and Process waters ............. 63

4.2.4.1. Nutrient solubilisation during the BMP test ........................... 69

4.2.5. Theoretical BMP v. Experimental BMP ....................................... 72

4.2.6. Energy production of the hydrothermal treatments. .................... 75

4.3. Conclusions ....................................................................................... 76

4.4. Summary ........................................................................................... 77

4.5. Publications and awards derived from this chapter ........................... 77

Chapter 5. Hydrothermal Carnonization of Sewage Digestate: Influence of

the solid loading on hydrochar and process water characteristics. .............. 79

5.1. Introduction ........................................................................................ 79

5.2. Methods ............................................................................................. 81

5.2.1.1. Mass and energy balance .................................................... 81

5.3. Results and discussions .................................................................... 83

5.3.1. Mass balance .............................................................................. 83

5.3.2. Hydrochar characteristics ........................................................... 84

viii

5.3.2.1. Physical characteristics ........................................................ 84

5.3.2.2. Elemental composition of the hydrochar............................... 87

5.3.2.3. Energy characteristics of the hydrochar ............................... 88

5.3.2.4. Carbon Balance .................................................................... 89

5.3.2.5. Nutrient balance ................................................................... 90

5.3.3. Characteristics of the process waters ......................................... 92

5.3.3.1. pH ......................................................................................... 94

5.3.3.2. Total Solids and Total Volatile Solids ................................... 94

5.3.3.3. Chemical oxygen demand and Total Organic carbon ........... 95

5.3.3.4. Volatile Fatty Acids (VFA’s) .................................................. 99

5.3.3.5. Phosphorus .......................................................................... 99

5.3.3.6. Nitrogen .............................................................................. 100

5.3.4. Anaerobic digestion and biomethane potential of process waters

(BMP).................................................................................................. 103

5.3.5. Maximum potential methane yields ........................................... 106

5.3.6. AD+HTC system analysis ......................................................... 107

5.4. Conclusions ..................................................................................... 112

5.5. Summary ......................................................................................... 112

5.6. Publications and awards derived from this chapter ......................... 113

Chapter 6. Evaluation and comparison of product yields and bio-methane

potential from hydrothermally treated sewage sludge. ............................... 114

6.1. Introduction ...................................................................................... 114

6.2. Results and discussions .................................................................. 116

6.2.1. Mass balance ............................................................................ 116

6.2.2. Characteristics of the process waters ....................................... 117

6.2.2.1. pH ....................................................................................... 117

6.2.2.2. Total Solids and Total Volatile Solids ................................. 120

6.2.2.3. Chemical oxygen demand and Total Organic carbon ......... 120

6.2.2.4. Volatile Fatty Acids (VFAs) ................................................. 122

ix

6.2.2.5. Phosphorus ........................................................................ 125

6.2.2.6. Nitrogen .............................................................................. 128

6.2.3. Anaerobic digestion and biomethane potential of process waters

(BMP).................................................................................................. 128

6.2.4. Hydrochar characteristics ......................................................... 133

6.2.4.1. Physical characteristics ...................................................... 133

6.2.4.2. Elemental composition in hydrochar ................................... 135

6.2.4.3. Energy characteristics in hydrochar ................................... 137

6.2.5. Energy balance ......................................................................... 137

6.3. Conclusions ..................................................................................... 140

6.4. Summary ......................................................................................... 140

6.5. Publications and awards derived from this chapter ......................... 141

Chapter 7. Mass and Energy Integration Study of Hydrothermal Processing

with Anaerobic Digestion of Sewage Sludge.............................................. 142

7.1. Introduction ...................................................................................... 142

7.2. Material and methods ...................................................................... 144

7.2.1. Process description ................................................................... 144

7.2.2. Mass and energy balance ......................................................... 145

7.2.2.1. Sludge and anaerobic treated sludge samples................... 145

7.3. Results and discussions .................................................................. 147

7.3.1. Mass balance ............................................................................ 147

7.3.2. Energy balance ......................................................................... 158

7.3.3. Economics ................................................................................ 161

7.4. Conclusions ..................................................................................... 166

7.5. Summary ......................................................................................... 166

Chapter 8. General Discussions ................................................................ 168

8.1. Introduction ...................................................................................... 168

8.2. Hydrochar valorisation ..................................................................... 169

x

8.2.1. Process water valorisation ........................................................ 174

8.2.2. Nutrients fate............................................................................. 180

8.3. Feasibility of hydrothermal treatment integration with AD ................ 183

Chapter 9. Overall Conclusions and Recommendations ............................ 185

9.1. Research conclusions ..................................................................... 185

9.2. Further research and considerations ............................................... 187

Refererences…………………………………………………………………….188

xi

List of figures

Figure 2.1.- Conventional WWTW with enhanced energy production. ......... 19

Figure 2.2.- WWTW with hydrothermal treatment after AD. ......................... 22

Figure 2.3.- Natural nitrogen versus reactive nitrogen in UK (Lillywhite and

Rahn, 2005). ................................................................................................ 27

Figure 2.4.- Process flowsheet of Enhanced Biological Phosphorus and

Nitrogen Removal (EBPR) wastewater treatment plant without sidestream

treatment (A) and with sidestream treatment by MAP process (B) (Münch

and Barr, 2001). ........................................................................................... 30

Figure 3.1.- Gas cromatograph Agilent 7890 A. ........................................... 34

Figure 3.2.- Thermal reactor used for the experiments. ............................... 35

Figure 3.3.- Elemental Analyser, CE Instruments Flash EA 1112 Series. .... 36

Figure 3.4.- BMP experiments. .................................................................... 36

Figure 3.5.- Biogas collection system. ......................................................... 37

Figure 3.6.- Experimental design for objective 1. ......................................... 42

Figure 3.7.- Experimental design for objective 2. ......................................... 44

Figure 3.8.- Experimental design for objective 3. ......................................... 46

Figure 3.9.- General process diagram for experimental design of objective 4.

..................................................................................................................... 48

Figure 4.1. Fate of Phosphorus (a), Nitrogen (b) and organic matter (c) after

hydrothermal processing of digestate samples (Control) for 30 min and at

160°C (5bar), 220°C (35bar) and 250°C (40bar). ........................................ 55

Figure 4.2.- Changes in soluble COD of Slurries (a) and Process Waters (b)

during BMP tests. ......................................................................................... 64

Figure 4.3.- Normalised VFA production from Slurries (a) and Process

Waters (b) during BMP tests. ....................................................................... 66

Figure 4.4.- Cumulative methane production from Slurries (a) and Process

Waters (b) during BMP tests. ....................................................................... 67

Figure 4.5. Changes in the concentrations of soluble TKN and ammonium in

slurries (a and c) and process waters (b and d) before (Day 0) and after (Day

21) BMP tests. ............................................................................................. 70

Figure 4.6. Changes in Total Soluble Phosphorus and Reactive Phosphorus

concentrations in slurries (a and c) and in process waters (b and d) before

(Day 0) and after (Day 21) BMP tests. ......................................................... 72

xii

Figure 5.1.- Changes in the feedstock after HTC at different solid loadings. a)

Product distribution in Liquid, Solid and Gas fractions and b) Fate of solids

from the feedstock ....................................................................................... 84

Figure 5.2.- Atomic H/C and O/C ratios of feedstock and hydrochars

following HTC (250°C and 30min retention time) at different solid loadings. 87

Figure 5.3.- Mas balance distribution of Phosphorus (a) and Nitrogen (b)

before and after HTC treatment at different solid loadings. .......................... 91

Figure 5.4.- Solubilisation of (a) carbon rich compounds (Chemical Oxygen

Demand (COD), VFAs (Volatile Fatty Acids) and Total organic carbon

(TOC)); (b) nitrogen rich compounds (Total Kjeldahl Nitrogen (TKN) and

Ammonium; (c) phosphorus rich compounds (Total Phosphorus (TP) and

Reactive Phosphorus (RP)); and (d) solids (Total Solids (TS) and Volatile

Solids (VS). .................................................................................................. 97

Figure 5.5.- Percentage of Nitrogen (a) and Phosphorus (b) extracted from

the original solids into process waters after HTC processing. .................... 102

Figure 5.6.- BMP test results (a) from process waters - PW at different solid

loadings and changes in COD (b) and VFA (c) concentration during BMP

tests. .......................................................................................................... 106

Figure 5.7.- Aspen diagram for the integration of the HTC process at the end

of a WWTW with a sludge of 20% of solids................................................ 111

Figure 6.1.- Mass balance solid distribution of the thermal treatments of

different sewage sludge at different temperatures. .................................... 117

Figure 6.2.- (a) pH and concentration of (b) Total Solids(TS) and Total

Volatile Solids (TVS), (c) Total Organic Carbon (TOC) (d) Chemical Oxygen

demand (COD) and (e) Volatile Fatty Acids (VFAs) of the different sewage

sludge’s process waters after the different thermal treatment. .................. 119

Figure 6.3.- Concentration of phosphorus and nitrogen of the different

sewage sludge after the different thermal treatment: (a), Total Phosphorus

and Reactive Phosphorus, (b) Total Kjeldahl Nitrogen (TKN) and Ammonia

and solubilisation of the c) Nitrogen and d) Phosphorus. ........................... 127

Figure 6.4.- BMP of the liquid fraction of the different sewage sludge prior

and after thermal treatment. ....................................................................... 131

Figure 7.1.- Mass and energy balance scenarios of the a) Primary Sludge, b)

Secondary Sludge and c) Mix Sludge at 160°C thermal treatment. ........... 154

Figure 7.2.- Mass and energy balance scenarios of the a) Primary Sludge, b)

Secondary Sludge and c) Mix Sludge at 250°C thermal treatment. ........... 157

xiii

List of Tables

Table 2.1.- Sewage sludge reuse and disposal routes – tonnes dry solids

(DEFRA, 2012b). ......................................................................................... 11

Table 2.2.- Different Pre-treatments for enhance the anaerobic digestion. .. 16

Table 2.3.- Different thermal treatments used for improve the biomass

characteristics .............................................................................................. 20

Table 3.1.- APHA analyses for feedstock characterisation .......................... 33

Table 3.2.- Gantt chart for the BMP analyses. ............................................. 37

Table 4.1.- Proximate and ultimate analyses of the feedstock (digestate) and

hydrochar. .................................................................................................... 57

Table 4.2.- Energy characteristics of hydrochar. .......................................... 58

Table 4.3.- Characterization of filtered digestate (Control liquor) and process

waters after HTP. ......................................................................................... 61

Table 4.4.- Comparisons between experimental BMP and theoretical BMP.75

Table 4.5.- Energy production of different thermal treatment configurations

for a 15% solids sewage sludge. .................................................................. 76

Table 5.1.- Proximate and ultimate analyses of the feedstock (digestate

cake) and hydrochar. ................................................................................... 86

Table 5.2.- Energy characteristics of the feedstock and hydrochars. ........... 89

Table 5.3.- Characteristics of the control and process waters from different

solids loading. .............................................................................................. 93

Table 5.4.- Comparison of the Experimental BMP v. theoretical BMP. ...... 107

Table 5.5.- Energy production and consumption per kg of feedstock. ....... 109

Table 5.6.- Energy balance of the HTC-AD integration scenario for 20%

solids of digestate sludge. .......................................................................... 110

Table 6.1.-Proximate analyses of the Process waters. .............................. 124

Table 6.2.- BMP, biogas composition and COD removal of the process

waters. ....................................................................................................... 132

Table 6.3.-Proximate analyses of the feedstock (control) and hydrochar. . 134

Table 6.4.- Ultimate analyses of the feedstock and hydrochar. .................. 136

Table 6.5.- Energy production and consumption per kg of feedstock.

Considering 20% of solids loading. ............................................................ 139

Table 7.1.- Process assumptions and calculation basis considered for the

mass and energy balances of the different scenarios. ............................... 146

xiv

Table 7.2.- Mass balance of the proposed scenarios. ................................ 149

Table 7.3.- Nitrogen and Phosphorus balance and struvite production of the

proposed scenarios. ................................................................................... 151

Table 7.4.- Energy balance of the proposed scenarios. ............................. 160

Table 7.5.- Potential economic benefits of integrating HTC with AD. ......... 163

Table 7.6.- Potential economic benefits of scaling up the scenarios. ......... 165

xv

List of Acronyms and Abbreviations

AD

ADPS

ADSS

ATH

BMP

CHP

COD

CSTR

DEFRA

EU

FIT

GEMA

HT

HTC

HTL

HTG

HTP

HHV

MAD

MSW

PW

PS

SS

ROI

SCOD

SVS

TCOD

Anaerobic digestion

Anaerobic digested primary sludge

Anaerobic digested secondart sludge

Advance thermal hydrolysis

Biomethane Potential

Combine heat power

Chemical oxygen demand

Continuous stirred reactor

Department of Environmental Food and Rural Affairs

European Union

Feed in Tariff

Gas and Electricity Markets Authority

Hydrothermal treatment

Hydrothermal carbonisation

Hydrothermal liquefaction

Hydrothermal gasification

Hydrothermal process

High Heating Value

Mesophilic anaerobic digestion

Municipal solid waste

Process water

Primary sludge

Secondary sludge

Return of investment

Soluble chemical oxygen demand

Suspended volatile solids

Total chemical oxygen demand

xvi

TKN

TS

VFA(s)

VS

UK

WWTW(s)

Total Kjeldahl nitrogen

Total solids

Volatile fatty acid(s)

Volatile solids

United Kingdom

Waste Water treatment work(s)

1

Chapter 1. Introduction

1.1. Background

Over the past decade, sludge management at Waste Water Treatment

Works (WWTWs) has been considered one of the biggest concerns for water

companies and environment protection agencies. In the UK, over 16 billion

litres of waste water per day are collected and treated in 9,000 WWTWs

before they are discharged to inland waters, estuaries or the sea (DEFRA,

2012a). That implies the need for suitable treatment processes to be carried

out in order to reduce potential risks to the environment and public health. As

a result of that, around 1.4 million tonnes (dry weight) of sewage sludge are

produced annually in the UK (DEFRA, 2012b).

Sewage sludge can be used for the production of energy due to its large

organic matter content (Kim et al., 2014). Anaerobic digestion (AD) has been

commonly used for sewage sludge treatment as this feedstock does not

need to be dried or dewatered before treatment, which reduces net

operational costs. In the UK, around 75% of the total sewage sludge

produced undergoes anaerobic digestion (DEFRA, 2012b). The main

purpose of the anaerobic digestion process is to stabilise the organic matter

present in sewage sludge before disposal and to produce bioenergy in the

form of methane to reduce net energy costs. Sewage sludge contains

complex biodegradable organic compounds that must be solubilised and

broken down into smaller monomers before being assimilated by anaerobic

bacteria (Gunnerson and Stuckey, 1986). According to Abelleira-Pereira et

al. (2015) and Hindle (2013) only one half of the organic matter in sewage

sludge is susceptible to anaerobic biodegradation, resulting in biogas

formation. Anaerobic digestion is considered as an economical and

sustainable technology for sewage sludge stabilisation, considering the

beneficial production of methane that can be used to produce electricity and

heat at WWTWs (Abelleira-Pereira et al., 2015, Hindle, 2013).

After anaerobic digestion, the treated sludge contained in AD reactors

(digestate) requires proper disposal. Currently, the main routes for the

disposal of digestate in the UK includes some pre-treatment processes to

2

reduce moisture (thickening, dewatering, centrifugation, filtration, etc.),

before final disposal on agricultural land (79%), incineration (18%) or

landfilling (0.6%) (DEFRA, 2012b).

However, the large quantities and characteristics of sewage sludge that are

being produced, treated and disposed, have induced changes into the

European Directive regarding requirements for sewage sludge application on

agricultural land. Currently, the EU Sludge Directive 86/278/ECC only limit

the presence of seven heavy metals for sewage sludge intended for

agricultural use and sludge treated-soils. According to Dichtl et al. (2007), the

European Commission is assessing whether the current Sludge Directive

should be revised in order to set additional requirements, including the

presence of organic compounds and more stringent limits for hazardous

substances, which will demand higher quality requirements for treated sludge

if the current disposal route to land is used. Because of these imminent

changes, it is expected that the disposal of sewage sludge and digestate on

land will no longer be accepted despite their valuable nutrient and organic

material content.

As a consequence, WWTWs will have to face the very difficult task of finding

alternatives to current sewage sludge treatment and final disposal routes and

therefore, there is a clear opportunity for developing innovative solutions that

simultaneously help to deal with this emerging challenge and delivering

sustainable targets set by the wastewater industry in terms of energy

efficiency, renewable energy generation and nutrient recovery. Furthermore,

the increasing amounts of sludge being produced in WWTWs encourage

researchers and engineers to pay more attention to particular aspects of the

current management of sewage sludge, especially considering the

opportunities for bioenergy generation and resource recovery and reuse. The

challenge here is to achieve an effective and sustainable approach delivering

three important targets: (a) reduce the amount of “waste” returning to the

environment; (b) generate an income stream from the recovery and reuse of

valuable resources embedded in waste streams; and (c) reduce the overall

treatment costs by considering the implications of new sewage management

3

options and the changes in the water, carbon and nutrient cycles within

WWTWs (Abelleira-Pereira et al., 2015).

Therefore, it is important to investigate new technologies capable of treating

sewage sludge and change perceptions about using sewage sludge as a

future energy resource (Almeida, 2010, Danso-Boateng et al., 2015, He et

al., 2013, Kim et al., 2014).

Hydrothermal processing is currently being considered as an alternative

technology to further harness energy from sewage sludge and digestate (He

et al., 2013, Zhao et al., 2014) and to reduce the issues related to current

disposal of final solid products. Hydrothermal processing involves the

treatment of biomass in hot compressed water and depending upon process

severity, can produce either a solid hydrochar, a biocrude or a syngas. The

main aim of the hydrothermal processing routes is energy densification,

which is produced largely by the removal of oxygen. Hydrothermal pre-

treatment can also be used to enhance the sludge solubilisation and

subsequent biogas production when processed by anaerobic digestion (Wirth

et al., 2015, Wang et al., 2010). Conventional Thermal Hydrolysis (TH) is

carried out at 170°C and produces a sludge that is more biodegradable than

the raw sludge (Shana et al., 2013). When it is applied at lower temperature

in the presence of hydrogen peroxide, it is referred to as Advanced Thermal

Hydrolysis (ATH) (Abelleira et al., 2012).

Depending on the temperature and pressure that it is applied, the products

from hydrothermal processes are different. At temperatures ranging from

200°C to 250°C, the process is referred to as HT carbonization (HTC) and

predominantly produces a solid biocoal like product called hydrochar; at

intermediate temperatures of approximately 250–375°C, the process is

known as HT liquefaction (HTL), primarily producing an oil referred to as

biocrude; at the higher temperature range (i.e., greater than 375°C), the

process is called HT gasification (HTG), predominantly producing a gas

product containing CO, H2 and methane (syngas). The hydrochar produced

from HTC can be co-fired with coal or used as soil amendment; the biocrude

from HTL can be upgraded to a variety of fuels and chemicals, while the

4

syngas from HTG can be used for combustion or converted to hydrocarbons

by either biological or catalytic processing (Biller and Ross, 2012).

It is known that the digestate (i.e., sewage sludge following anaerobic

digestion) still has large amounts of organic matter (Kim et al., 2014) and

converting this organic matter by hydrothermal carbonisation into bio-coal

may be possible, which in return would bring alternative disposal routes to

digestate. Hydrothermal processing also generates a “process water" that is

rich in organic compounds and cannot be directly disposed into the

environment (Almeida, 2010, Becker et al., 2014, Kim et al., 2014, Stemann

et al., 2013, Wirth et al., 2015, Zhao et al., 2014). The treatment of this

“waste stream” is essential and it has been proposed that it can be treated

anaerobically enhancing net biogas yields.

According to Mumme et al. (2015) and Sridhar Pilli et al. (2015) the

integration of the HT step into the waste water systems is suggested to be

energy positive. In fact, CAMBI® and BIOTHELYS ® are commercial high-

temperature processes that have been successfully developed as pre-

treatment steps for hydrothermal hydrolysis of sewage sludge, resulting in

extra methane production to up to 43%, when compared with conventional

AD processes without pre-treatment (Sridhar Pilli et al., 2015). However, HT

as a post-treatment step after AD is an approach that is still under research

and development, but preliminary findings have shown that this approach

could be even more effective with regard to overall energy production from

sewage sludge. Aragón-Briceño et al. (2017) found that thermal treatment of

sewage sludge as a post-treatment step can improve the overall energy

production up to 179% compared with the 43% extra energy of the thermal

hydrolysis as pre-treatment. Therefore, further research on process

conditions and overall benefits from hydrothermal processes as a post-

treatment step after AD is still needed.

In this research project, it is considered that the use of Hydrothermal

Treatments is not only a suitable option to effectively handle sewage sludge,

considering future vetoes on sludge-to-land practices, but it can also help to

obtain valuable by-products (i.e., biochar, bio-oils, syngas, bio-fertilisers,

etc.).

5

1.2. Aim, scope and objectives

Due to the increasing amount of digestate produced in WWTWs, and a

potential ban on the current final disposal route on agricultural land, there is

a need to look for options aimed at reducing operational costs at WWTW,

including digestate stabilisation and disposal, by delivering a sustainable

approach. Therefore, the aim of this project is to assess alternatives to

enhance the way sewage sludge and digestate is handled in WWTWs, by

focusing on the use of hydrothermal processes and the potential of

recovering energy and nutrients. The scope of this project is to assess at lab

scale such alternatives by introducing hydrothermal processes in sewage

sludge management in modern WWTWs.

The specific objectives for this research project are:

To evaluate the effect of temperature during HTC processing

conditions of sewage digestate on product yields and the

characteristics of the different by-products.

To evaluate the influence of solid loading on hydrochar and process

water characteristics from HTC of sewage digestate.

To investigate the changes that occur in sewage sludge samples

collected at various stages along treatment process units in a

conventional WWTW, when subjected to hydrothermal processes at

different temperatures.

To assess the integration of HTP with AD through mass and energy

balances from proposed process configurations from different sewage

sludge based on the results obtained from experimental analyses.

1.3. Structure of Thesis

This thesis is organized in nine chapters with the introduction section

constituting the first chapter. In this chapter the background, aim, scope and

objectives are highlighted. Chapter 2 presents a thorough literature review

that focuses on the problematics of sewage sludge management in the UK,

6

AD as a common option to deal with sewage sludge and opportunities from

hydrothermal treatments as potential processes to be integrated with the AD.

Chapter 3 describes the general methodology followed for all the

experiments carried out. Nevertheless, more detailed methodology is

included in each result chapter.

In chapters 4 to 7, the results are reported for each stage of this project.

Every chapter has been written following a style similar to journal papers and

hence, they contain several sections including an introduction, materials and

methods, results and discussions, conclusions, summary and list of

publications and awards derived from each chapter.

Chapter 4: This chapter “Evaluation and comparison of product yields and

bio-methane potential in sewage digestate following hydrothermal treatmen”t

is related with objective 1. This research investigates the effect of process

temperature on the characteristics of hydrochars and process waters from

hydrothermal processing of sewage digestate and compares the yields and

characteristics of the different products including the fate of nitrogen and

phosphorus species. In addition, experimental biomethane potential (BMP)

tests were conducted on process waters on their own and in combination

with hydrochars to assess the effect that hydrochars may have on AD

processes. The results from experimental BMP tests were compared to

theoretical predictive models.

Chapter 5: “Hydrothermal Carbonization of Sewage Digestate: Influence of

the solid loading on hydrochar and process water characteristics” is related

with objective 2. In this chapter the influence of solid loading on the

composition of the resulting hydrochar and process water from sewage

digestate is presented. An evaluation of product yields, solubilisation of

organic carbon and biomethane potential of the process water is compared

for 2.5-30% solid loadings at a HTC temperature of 250°C with a 30-minute

reaction time.

Chapter 6: “Evaluation and comparison of product yields and bio-methane

potential from hydrothermally treated sewage sludge” is related with

objective 3. In this chapter the potential of hydrothermal processing as a

7

novel alternative for sewage sludge treatment was evaluated. Primary,

secondary and digestate sludge were treated using hydrothermal processes.

The effect of process temperature was evaluated with regard to product

yields, biomethane potential and solubilisation of organic carbon and

nutrients. Tests at 160 and 250°C for 30-minute reaction time were carried

out.

Chapter 7: “Mass and Energy Integration Study of Hydrothermal

Carbonization with Anaerobic Digestion of Sewage Sludge” is related with

objective 4. In this chapter the potential of integration of HTC with AD for

sewage sludge treatment was evaluated. Mass and energy balances were

carried out from six proposed process configurations from different sewage

sludge and digestates (primary, secondary and 1:1 Mix) in order to evaluate

the waste generation, nutrients potential fate, net energy production and

potential profit.

Chapter 8 contains a general discussion that summarizes research findings

and a critical analysis against published research in the field. Finally, Chapter

9 presents the general conclusions from this research work and

recommendations for further studies.

8

Chapter 2. LITERATURE REVIEW

2.1. Water Supply in the UK

According to the Drinking Water Inspectorate (2014), there are 53 million

people benefiting from water supply services in the UK. That effectively

means a total drinking water production of 13,707 million L/day, which is

supplied by water treatment plants across the UK. That water comes from

different sources including surface waters (64.1%), groundwater (30.1%) and

others considered mixed sources (5.8%) (DEFRA, 2012b)

The main role of water companies in the UK is to collect, clean and deliver

safe drinking water to their customers and to collect and clean waste water

before returning it to the environment.

One of the main ways in which water sources can be affected is by the

amount of water abstracted to meet the increasing demand from the UK’s

fast growing population, and by the quality of the discharged effluent from

wastewater treatment systems. “Pollution imposes not only environmental

costs through its effect on aquatic life, but also financial costs from the

treatment of water for drinking. The accumulative cost of water pollution in

England and Wales has been estimated at up to £1.3 billion per annum”

(NAO, 2010).

According to NAO (2010) , water pollution derives from two sources:

1. Point Source Pollution: It comes from a single identifiable source such

as a factory or sewage treatment works.

2. Diffuse pollution: It is caused by excessive or improper use of

fertilisers, poor management of waste or livestock on farms, the run-

off of chemical from light industry or wrongly connected domestic or

commercial drainage systems. It is very difficult to identify where the

pollution is coming from the agricultural sector is considered the major

contributor of diffuse pollution, but urban sources contributes to

diffuse pollution too.

On average, 80% of all drinking water supplied to UK households will

become domestic wastewater, and in addition to trade wastewater, it is

9

expected that the production of sewage and sewage sludge will continue

increasing as a consequence of population and economic growth (NAO,

2010).

2.2. European Water Framework Directive (WFD)

The European Water Framework Directive came into force in December

2000 and became part of UK law in December 2003. It consolidates a

number of pieces of EU legislation.

The directive is designed to help, protect and enhance the quality of:

1. Surface freshwater (including lakes and rivers).

2. Groundwater

3. Groundwater dependant ecosystems

4. Estuaries

5. Coastal waters out to one mile from low-water.

The specific goal of the WFD is for all EU member states to achieve “good”

ecological and chemical status for these water courses (Servern Trent

Water, 2013).

From the point of view of the UK Water Industry, the requirements of the

European Framework are getting more stringent every day, the investment in

maintenance is increasing and the profit is getting tight. This is reflected in

higher bills for customers, increasing carbon emissions and higher debts to

the companies and customers. Therefore, UK water companies have

recognised the need to increase the efficiency of their water treatment

systems and the production of renewable energy. To continue delivering

effective services, the UK water sector has also identified the need to

improve their processes through much greater innovation (Servern Trent

Water, 2010).

According to Servern Trent Water (2010), the EU framework policies do not

consider:

1. Sufficient account of the impact of carbon emissions or costumer bills.

2. Supply issues are addressed using regionally focused, capital

intensive solutions. It means that current regulatory framework

10

encourages the companies to look for a new water sources because

the demand is increasing.

3. Economic regulation no longer provides the right incentives. This

affect the investment in innovation in the water companies because

there are not sufficient money to encourage companies to create new

technology. The companies have tended to apply standards, capital-

intensive solutions to meet regulatory requirements because that’s

represent the “cheaper” option in the short term. It is not sustainable

for them invest in long term solutions despite the fact that it may be

profitable.

This make believe that now is a critical time for the UK Water Sector with a

stake in the industry to question what future direction they should take.

Without significant changes to the policy and regulatory framework the sector

does not look sustainable. While the framework has delivered higher

customer and environmental standards, the consequences have been

significant water company debt, higher bills to customers and increased

carbon emissions (Priestley, 2015).

In England, 35% of rivers achieved a good or very good status in 2017 under

the actual Water Framework Directive, lightly lower compared from 36% in

2012 (DEFRA, 2018). In 2010 in England, when new regulations

implemented by the EU Drinking Water Directive for private supplies were

introduced, 9.6% of water treatment plants did not pass the tests on public

water supplies. In 2013 however , only 0.3% of tests on public water supplies

failed to meet both EU and National Standards, due the new technologies

that have been implemented (Drinking Water Inspectorate, 2014). However,

the European Water Framework Directive requires Member States to

achieve “good status” in all natural bodies by 2027. This will not be possible

to achieve using current technologies and the strict standards but instead,

UK government aimed to set out a longer term goal by 2050 (Priestley,

2015). According to DEFRA and The Environmental Agency (2018), the UK

water companies will spend over £5 billion to benefit the natural environment.

Neverthless, an assessment made by The Environmental Agengy (2015)

showed that cost for meeting the WFD goals, that will benefit from preventing

11

deterioration and improving the water environment, would be around £23

billion.

2.3. Sewage Sludge in the UK

According to DEFRA (2012b), about 11 billion litres of waste water in the UK

were collected and treated in 9,000 WWTWs before the effluent was

discharged to inland waters, estuaries or the sea. That implies the need for

suitable treatment processes to be carried out in order to avoid potential

damage to the environment and public health problems.

The treatment of waste water has the objective of returning cleaner water to

the environment. As a consequence large quantities of sewage sludge are

generated. The sewage sludge comes from the organic matter used in

treatment process or biosolids removed from the waste water being treated.

Sewage sludge contains organic matter (i.e., carbohydrates, fats, proteins,

faecal material, etc.) and chemicals. (DEFRA, 2002, DEFRA, 2012b).

In the past, part of the sewage sludge was discharged to surface waters or

into the sea. However in 1998, the European Directive required the cessation

of these practices and made a call to find and use alternatives to re-use or

dispose of sewage sludge (DEFRA, 2012b). The changes to re-use and

disposal routes are shown in Table 2.1, where the baseline of 1992 is

contrasted with the situation in 2008 and 2010.

Table 2.1.- Sewage sludge reuse and disposal routes – tonnes dry solids (DEFRA, 2012b).

Reuse or

Disposal

Route

Sludge Discharged to Surface Waters Sludge Reused Sludge Disposed Total

Pipelines Ships Others Soil &

Agriculture Others Landfill Incineration Others

1992 8,340 273,158 - 440,137 32,100 129,748 89,800 24,300 999,673

2008 - - - 1,241,639 90,845 10,882 185,890 1,523 1,530,779

2010 - - - 1,118,159 23,385 8,787 259,642 2,863 1,412,836

One of the most commonly used alternatives for sewage sludge

management is spreading on agricultural land, because the sludge can be

used as an alternative soil building-material and fertiliser due to its

phosphate content and for being a source of slow– release nitrogen for land

restoration (DEFRA, 2012a). Nevertheless, the big quantities of sewage

12

sludge that are being produced and applied have induced changes into the

European Directive regarding requirements for sewage sludge application on

agricultural land. Because of those changes, it is expected that the disposal

of sewage sludge on land will no longer be accepted despite its nutrient and

organic material content, as lower limits for hazardous substances and

higher quality requirements in general are likely to be imposed. With these

new restrictions, sewage sludge will hardly be suitable for agricultural reuse

(Dichtl et al., 2007). That makes think of new alternatives to deal with

sewage sludge in WWTWs.

The focus is on alternative sewage sludge treatment technologies that gain

the most economical and ecological benefit from the sludge’s valuables.

Several technologies for nutrient recovery, especially phosphorus have been

developed.

2.3.1.Sewage Sludge Management

The large amount of sewage sludge generated at WWTWs, has made its

treatment an important issue not only in the UK, but also worldwide.

However, any approach to sludge management always needs to consider

legal boundaries and operational costs before making a decision about the

selected disposal method.

Today around 1.4 million tonnes (dry weight) of sewage sludge are produced

annually in the UK. The disposal methods include spread on farmland (58%),

incineration (16%) and power generation via gasification (3.5%) and others

(22.5%) including direct application on forrests, compost or another methods

(BIOMASS Energy Centre, 2011).

Disposal methods in the UK for sewage sludge are described as follows

(ISWA and EEA, 1997, Thames Water Ltd, 2008):

Agricultural use: The main objective is to utilise nutrients such as

phosphorus and nitrogen and partly to utilise organic substances for

soil improvement.

Composting: Composting aims to stabilize biologically the sludge

controlling pollution risks in order to develop agriculture or other end

13

use outlets exploiting the nutrient or organic value. Composting

involves aerobic degradation of organic matter, as well as a potential

decrease of the sludge water content, the efficiency of which depends

on the composting process. Is considered a valuable soil improver.

Incineration: The process is done at high temperatures (over 800

Celsius degrees) and consists of burning the waste and recovering

some heat to reuse in the process. The waste generated is ash that

mainly consists of heavy metals.

Landfilling: the process consists of placing the sewage sludge in the

landfill as layers between each level. Landfilling will have the lowest

priority in the waste hierarchy and will only be chosen when no other

ways to dispose of the sludge exist. It is not an option when the place

has a vulnerable geologic media.

Moreover the sewage sludge can be dried and used for energy generation.

Methods like combustion, gasification, pyrolysis and anaerobic digestion are

often used. However AD is most common because does not need the sludge

to be dried or dewatered before treatment (less operation costs).

Researchers like Danso-Boateng et al. (2015) and He et al. (2013) mention

that sewage sludge has attracted great attention as a promising feedstock for

the production of renewable biofuels.

2.4. Anaerobic Digestion in UK

In last years, the number of researchers studying anaerobic digestion has

increased due to its potential to support the production of valuable products

in a biobased economy. The AD treatment of organic wastes decreases the

amount of organic solids for final disposal and it is considered a clean

technology based on its capacity to support bioenergy production (Wang et

al., 2010). That shows the importance of this technology in waste water

treatment (Cano et al., 2014, Hindle, 2013). A wide range of wastes are

susceptible to being degraded anaerobically, as it is reported by Carlsson et

al. (2012): municipal wastes, organic wastes from food industry, energy

crops, agricultural residues, manure and waste water treatment plant

residues.

14

Conventional AD brings economic and environmental benefits. One of the

advantages of anaerobic digestion is the production of methane, which can

be used as a source of energy for the production of electricity and heat.

Furthermore, it can be used just like “natural gas” in many other applications

(Abelleira-Pereira et al., 2015, Hindle, 2013).

AD of solid residues is commonly practiced in municipal waste management

to stabilize organic waste, reduce solid volume and at least partially disinfect

solids prior to disposal. Many AD facilities have the added benefit of energy

recovery via methane production (Elliott and Mahmood, 2007, DEFRA,

2012b). The majority of anaerobic digesters operating in the municipal sector

use single phase mesophilic reactors (Erdal et al., 2006). The use of

thermophilic digesters has become more attractive due to their performance,

better pathogen destruction and higher digestions rates, which allow the

anaerobic digestion facilities to operate at higher loading rates with smaller

reactor volumes (Erdal et al., 2006). Thermophilic digestion can reduce the

amount of difficult-to-degrade organic materials, thus improving the overall

removal efficiency of organics. Negative aspects of thermophilic digestion

include increased operator attention, higher odour release potential, higher

susceptibility to process upsets and poorer quality of dewatering filtrate

(Erdal et al., 2006, Tchobanoglous et al., 2003). Two stage digestion

systems, which segregate the formation of volatile fatty acids from

methanogenesis, have also been developed, improving the overall digester

performance (Shana et al., 2011).

According to ADBA (2015) the AD sector in the UK grew 33% from 2013 to

2014. It means that by 2014 in UK there were around 150 non wastewater

anaerobic digesters plants and 250 anaerobic digesters plants serving

WWTWs. Moreover, DEFRA (2012b) reported that 75% of sewage sludge

generated from treatment processes undergoes anaerobic digestion.

Anaerobic digestion technology for sewage sludge in wastewater treatment

plants (WWTWs) has been widely spread for decades (Cano et al., 2014).

During the anaerobic digestion process, sludge constituents are solubilised

by bacterial action and accumulated in the aqueous phase (i.e., soluble

chemical oxygen demand (COD) increases). Soluble COD is in turn

15

fermented into volatile fatty acids (VFAs), which are ultimately converted into

biogas (i.e., methane and carbon dioxide) by methanogens. This highlights

the importance of considering sewage sludge as a process by product that

can be considered as an income stream for WWTWs. In fact, there are a

number of practical examples in which better use of sewage sludge can be

made (Servern Trent Water, 2013):

Energy generation: Water companies can produce around 200GW/h

of electricity from sewage sludge, which is about 25% of their total

needs.

Fertiliser production. Sewage Digestate is rich in phosphorous and

nitrogen and, when treated, it can be used as a secondary source for

commercial fertilisers.

Phosphorous production. Phosphorous is a scare resource and is

used in many other products as well as fertiliser, for steel production

and in the manufacture of some detergents.

In summary, recent escalation of energy costs and technical advances in

anaerobic technology have subsequently made anaerobic digestion one of

the most cost/effective alternatives to sewage sludge disposal, particularly

because latest technological advances hold the potential for higher methane

recuperation while using smaller reactors (Elliott and Mahmood, 2007).

2.4.1.Anaerobic Digestion Pre-treatments

In order to improve the AD performance, various technologies has been

developed as pre-treatment of the sludge. The benefits of sludge

solubilisation prior to anaerobic treatment are twofold; Firstly, the increase in

the amount of released soluble substrate significantly increases VFA

generation for subsequent improved gas production and secondly,

pretreatment reduces the viscosity of the sludge, enabling a greater solids

concentration to enter an anaerobic reactor. Higher feed solids either result

in increased digestion times in an existing reactor or allow for a smaller

reactor volume (Elliott and Mahmood, 2007).

As it is showed in the Table 2.2, most of pre-treatments enhance the

solubilisation of organic matter (COD), volatile solids reduction and gas

16

production. However, there are not a pre-treatment method that can be

determined as being the best all round solution.

Table 2.2.- Different Pre-treatments for enhance the anaerobic digestion.

* Elliot and Mahmood (2007)

Pre-treatment Principle Effect

Ultrasound The process consists in appling high-frequency sound waves

(generated by a vibrating probe) that makes that the cell walls

ruptured due the pressure drop below the evaporating pressure

forming gas bubbles. As a result of the gas bubbles, the

temperature and pressure gradients increments in the liquid phase

which ruptures cell membrane, releasing intercellular matter in the

bulk solution.*

COD removal: 11-

39%.Volatile solids reduction:

54% (Khanal et al., 2006).

Gas production increment:

17% (Muller et al., 2003).

Thermal It is the exposure of the sludge to high temperatures (105-200°C to

enhance the cellular disintegration and thus reduces the time

required for hydrolysis step in the anaerobic digestion process.*

COD removal: 60-

71%.Volatile solids reduction:

36-59%. Gas production

increment: 54-92% (Valo et

al., 2004).

Ozone oxidation Mechanistically ozone reacts with polysaccharides, proteins, and

lipids (which are components of cell membranes), transforming

them into smaller molecular weight compounds (Bablon et al.,

1991). In doing so, the cellular membrane is ruptured, spilling the

cell’s cytoplasm. If the ozone dose is sufficiently high,

mineralization of the released cellular components could also occur

(Elliott and Mahmood, 2007).

Volatile solids reduction is

about 56% (Sievers et al.,

2004).

Alkaline Heo et al (2003) demonstrated how alkali addition alone is capable

of solubilizing Sludge.*

COD solubilisation between

28-38% and gas production

was increased between 66-

88% (Heo et al., 2003).

Mechanical The hydrolysis of cellular membranes can also be achieved by

mechanical rupturing techniques. The two predominant techniques

used are the Kady mill, which uses two counters rotating plates to

produce shear (Increases the soluble COD in a 25%), and the wet

milling, which is more of a grinding method.*

COD solubilisation until 25%.*

Enzymatic Enzymes: these products are used for accelerate the cellular

degradation.*

17

2.5. Thermal Hydrolysis

AD presents two main concerns in the process: low yield of the organic dry

solids degradation efficiency (less than 30-50%) and low methane production

at mesophilic conditions (Appels et al., 2011, Hindle, 2013, Ruiz-Hernando et

al., 2014, Schievano et al., 2012, Strong et al., 2011, Weiland, 2010).

It is known that the methane production and the organic dry solids

degradation are directly related with the methanogenic process. The

methanogenic process is limited by the hydrolysis rate of organic matter (i.e.,

flocs, micro flocs, aggregates of extracellular polymeric substances,

recalcitrant compounds of proteins and lipids, as well as component of hard

cell walls) and this rate could be limited when the hydraulic retention time is

low. That’s increase the risk of washing out the methanogens population

from digesters (Abelleira-Pereira et al., 2015, Carballa et al., 2011, Cano et

al., 2014, Shana et al., 2013, Strong et al., 2011). Furthermore, the

infrastructure has high costs and represents an obstacle for AD

development. For these reasons many researchers are looking to improve

the methane production throughout the enhancing of the hydrolysis rate

(Abelleira-Pereira et al., 2015, Hindle, 2013).

The recognition of the sludge hydrolysis stage as being the main rate limiting

factor in anaerobic digestion of sewage sludge has led to the development

and application of sludge pre-treatment technologies and thus the

intensification of the process (Shana et al., 2013).

The most widespread pre-treatment for AD used in Europe is the Thermal

Hydrolysis Process (THP) where sludge is heated to about 170°C and 7 bar

pressure for about 30 min and then anaerobically digested (Shana et al.,

2013). The aim of the THP is to break the long chain bonds of organic

compounds to improve the physical and chemical properties of the sludge to

be digested in the AD process (Wang et al., 2010). In addition, THP is used

to accelerate the hydrolysis step leading to high solubilisation, pathogen

reduction, good dewaterability and increase biogas production. The energy

input needed for the hydrolysis process is thermal energy and could be

satisfied from the energy production of the process, resulting in an

18

energetically self-sufficient process (Abelleira-Pereira et al., 2015, Pérez-

Elvira et al., 2008).

The study of Cano et al. (2014) determined that a proper energy integration

design could lead to important economic savings (5€/ton) and TH can

enhance up to 40% the income of the digestion plant, even doubling them

when digestate management costs are considered. Moreover, THP

increases the methane production up to 50% and makes the sludge

digestion process more tolerant to organic matter shock load and improves

sludge volatile reduction (VSR) from 30-50% to 50-60% (Cano et al., 2014,

Panter, 2008).

Perez-Elvira et al. (2010) reported 40% higher yield of biogas form the

system TH+AD than from other conventional. Studies by Donoso-Bravo et al.

(2011) reported 55% higher yield of biogas with TH + AD configuration.

Abelleira-Pereira et al. (2015) did a study of THP as pre-treatment and

reported that THP improval the volatile solids removal (37.6%) and the net

electricity production would be over 20% higher than conventional AD.

Other researchers studied THP as an intermediate digestion step concluding

there was an enhancement on the already digested sludge organic matter

degradation, sludge mass reduction and biogas production (Shana et al.,

2013). Shana et al. (2011) also showed that the novel intermediate THP

configuration produced 20% more biogas compared to THP configuration

(MAD + ITHP + MAD) with around 62% methane composition and 66%

volatile solid reduction.

One of the most common commercially available thermal processes used is

the CAMBI process developed by a Norwegian company, Cambi. This

process involves heating sludge to 165 °C for 30 min (see Figure 2.1) in

which the biogas production increases as a result of 60% VS reduction.

Other benefits of this process are the solid dewatering improvement and the

increase of the digester capacity as a result of the lower viscosity of

processed solids (Panter and Kleiven, 2005). The CAMBI process uses live

steam to preheat the sludge to 100 °C minimizing operational and corrosion

problems (Elliott and Mahmood, 2007, Weisz and Solheim, 2009).

19

Figure 2.1.- Conventional WWTW with enhanced energy production.

2.6. Hydrothermal processes

Thermal treatments have been used mainly for improving the feedstocks

characteristics as they hydrolyse the feedstock to improve the methane

generation or increase the energy densification producing chars, bio-oils or

syngas.

In Table 2.3, different thermal processes are listed with their respected

process conditions and main product. Thermal treatments are commonly

used to upgrade the characteristics of the biomass converting them into high

energy density products. Thermal treatments such as torrefaction and

pyrolysis are carried out in free oxygen and water conditions and

temperatures ranging from 200 to 300°C and 500 to 1000°C respectively

(Chen et al., 2015, Ronsse et al., 2013, Lee et al., 2012, Williams and

Besler, 1996).

The hydrothermal treatments are carried out in the presence of water at high

pressure and temperatures. The main by-product will depend mostly on the

temperature and pressure conditions. Conventional Thermal Hydrolysis (TH)

is carried out at 170°C and produces a sludge that is more biodegradable

than the raw sludge (Shana et al., 2013). The process refers to hydrothermal

carbonization (HTC) when temperatures range from 200°C to 250°C, and

20

predominantly produces a solid biocoal like product called hydrochar; when

temperatures range from 250°C to 375°C, the process is known as

hydrothermal liquefaction (HTL) producing mainly an oil referred to as

biocrude; when temperature range is greater than 375°C, the process is

called hydrothermal gasification (HTG) and predominantly produces a gas

product containing CO, H2 and methane called syngas. These by products

can be used as fuel sources to produce more energy (Biller and Ross, 2012).

Table 2.3.- Different thermal treatments used for improve the biomass characteristics

Thermal Process

Observations

Process conditions for biomass

Main product

References Temperature

range Pressure

Slow pyrolysis Limited or free of Oxygen. 500 1 atm Char Williams P, 1996 and Ronsse et al 2013

Fast Pyrolysis Limited or free of Oxygen. 650-1000 1 atm Bio-oil Williams P, 1996 and Ronsse et al 2013

Torrefaction Absence of oxygen. 200-300 1atm Char Lee et al, 2012 and Chen et al 2015

HTP In presence of water. Up to 180 1atm Hydrolized

sludge Shana et al., 2013 and Sridhar et al., 2014

HTC In presence of water. 200-250 10-40bar Char Danso Boateng, 2015 and Aragón-Briceño et al., 2017.

HTL In presence of water. 280-370 10-25Mpa Bio-crude Toor et al 2011 and Ekpo et al. (2015)

HTG In presence of water. greater than

370 25Mpa Syngas

Biller and Ross 2014 and Kruse et al 2005

According to Almeida (2010), there is a change in the perception of sewage

sludge because researchers consider that sewage sludge has the potential

of be an interesting energy resource. It is known that the sewage sludge has

great amounts of organic matter. Adopting this fact, that organic matter

content in the digestate can be harnessed to produce by-products

(hydrochar, bio-oil or syngas) that can be used as fuel sources. Although, the

process waters coming from hydrothermal processes are rich in organic

compounds and have the potential to be digested in an anaerobic reactor

(Aragón-Briceño et al., 2017). Therefore, hydrothermal processes have been

considered as alternatives technologies to develop to harness energy from

sewage sludge (He et al., 2013).

21

The Thermal pre-treatment know as hydrothermal hydrolysis, has been

shown to be a feasible, well established and commercially implemented

technology which helps to reduce volatile solids (VS) during AD, improve

biodegradability (BD), increase the dewaterability, increases up to 43%

methane production and produces a class A biosolid (Pilli et al., 2015).

Companies like Veolia and CAMBI have successfully developed the pre-

treatment steps for hydrothermal hydrolysis (See Figure 2.1) (Aragón-

Briceño et al., 2017). Nonetheless Hydrothermal treatment as a post-

treatment step after AD is a novel approach that is still under research and

development, but preliminary findings have shown that this approach could

be even more effective with regard to overall energy production from sewage

sludge – i.e., thermal hydrolysis can help to produce as much as 179% more

energy when placed as a post-treatment step than when used as a pre-

treatment step for AD (See Figure 2.2) (Aragón-Briceño et al., 2017).

A range of different solid wastes have been studied by hydrothermal

treatments (e.g., municipal solid wastes, agricultural wastes, industrial

wastes, etc.), but most of the studies covering hydrothermal treatment of

sewage sludge digestate have focused on the characterisation of the

resulting products (Berge et al., 2011a, Danso-Boateng et al., 2015, Escala

et al., 2013, Kim et al., 2014, Nipattummakul et al., 2010). Less focus has

been applied on the anaerobic digestion of the liquid products following

hydrothermal treatment (Wirth et al., 2015, Wirth et al., 2012). However, still

there is not much information about sewage sludge despite of having the

potential to be a feedstock material for thermal treatments for its high

hydrocarbons and inorganics compounds contents (Nipattummakul et al.,

2010).

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Figure 2.2.- WWTW with hydrothermal treatment after AD.

2.6.1.Hydrothermal Carbonization (HTC)

HTC is carried out at temperatures between 150 to 200 °C and for different

retention times. The process consists in concentrate the carbon in a stable

and easy handable material. Furthermore upgrade the poor fuels into higher

energy density solid fuels. The main product is the hydrochar, and is

reported to have good nutrient properties for terrestrial plants and has been

proposed as a source for soil amendment and also has the potential to be

co-fired with coal. The hydrochar is a novel material that has been probed in

many applications as a water purification material, fuel cell catalysis, energy

storage, CO2 sequestration, drug delivery and gas sensors (Biller and Ross,

2012, Danso-Boateng et al., 2015, He et al., 2013).

The biochar has H/C and O/C ratios comparable to that of low-grade coal but

a higher calorific value than such coals and for that reason can be used as a

potential fuel source. The aqueous products from HTC contain a lot of

organic compounds such as furans, phenols, acetic acid, and other soluble

organic compounds (Danso-Boateng et al., 2015). Because of that, the

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aqueous phase rich in organic compounds represents an issue and at the

same time a challenge to be solved to avoid harm to the environment.

The advantages of applying HTC to sewage sludge is that it produces an

extra value product (biochar) and sanitisation of the sludge (Catallo and

Comeaux, 2008, He et al., 2013).

There are some authors that have done studies with higher temperatures

than 200 °C and they still call it as HTC (Danso-Boateng et al., 2015, He et

al., 2013). In the study carried out by Danso-Boateng et al. (2015), the

authors concluded that the amount of carbon retained in hydrochars coming

from primary sludge decreased as temperature and time increased with

carbon retentions of 64–77% at 140 and 160 °C, and 50–62% at 180 and

200 °C. Increasing temperature and treatment time increased the energy

content of the biochar from 17 to 19 MJ/kg but reduced its energy yield from

88% to 68%. He et al. (2013) recovered in the hydrochar 88% of carbon and

removed 60% of nitrogen and sulphur.

2.6.2.Hydrothermal Liquefaction (HTL)

Hydrothermal Liquefaction of biomass consists of the conversion of biomass

into liquid fuels and chemical by applying high temperatures and pressures

for sufficient time to break down the solid biopolymeric structure of the liquid

components (Elliott, 2011). As mentioned previously, the HTL is carried out

at intermediate temperatures of approximately 200–375°C, primarily

producing oil called biocrude that can be used as a biofuel (Biller and Ross,

2012). That means a good advantage over the conventional incineration

because represents it has a solution to the sludge disposal problems, plus an

economical benefit (Itoh et al., 1994).

However the products of HTL not only are composed by oils, also has a

water fraction that is rich in organics, gas fraction and solid fraction which

can be used as fuel sources as well.

Actually there are many companies in Europe interested in developing and

commercializing the technology of HTL. Nevertheless there are a lot of points

to solve before as generate and standardize the process as is the optimal

conditions and what to do with the wastes of the process.

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HTL in sewage sludge has been studied also in many countries such as

USA, UK and Japan because of its environmentally friendly approach.

Biocrude can be produced from the dewatered sludge under conditions of

300°C and 10 MPa. However there are no well defined or ideal operational

conditions for HTL because the different types of sludge and this affect the

biocrude production (Liu et al., 2012).

2.6.3.Hydrothermal Gasification (HTG)

HTG is another promising thermal treatment for its viability, efficiency and for

its clean conversion of wastes into energy with minimal impact. So mainly the

HTG can add value to the wastes by transforming them into a low or medium

grade heating fuels (Nipattummakul et al., 2010).

HTG is carried out at temperatures greater than 375°C which it

predominantly produces synthetic gas (syngas). Syngas is mainly composed

of H2, CO, CO2, CH4 and light hydrocarbons, and the variability in

composition will depend on the reaction conditions. The H2 production is

favoured at temperatures greater than 500°C and below this temperature

CH4 production is favoured (Biller and Ross, 2012).

That means that the sewage sludge due its high content of organic material

and organic compounds, considered a good target to produce a clean fuel

with HTG. HTG is a process that can reduce the amount the volume of the

solid residue while is producing syngas and oil.

2.7. Nutrients pollution in wastewater

Nutrient pollution is one of the most widespread around the world. The

excess of nitrogen and phosphorus cause environmental problems that are

hard to deliver and expensive to remedy.

Nitrogen and phosphorus are widespread in nature (air, soil and water) and

help to the growth of algae in aquatic life, which provide food and habitat for

sea life. However in great amounts, the algae will grow a lot and will be

harmful to the environment, reducing the amount of oxygen in the water and

sometimes producing toxics that affect directly to the sea life (EPA, 2015).

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According to the EPA (2015), the nutrient pollution sources can come from

agriculture, storm water, waste water, fossil fuels and at home.

It is known that the waste water, especially the sludge, is rich in nutrients

such as nitrogen and phosphorous that are valuable and useful along the

organic matter when the soils are depleted or subject to erosion. That

means, the sewage sludge has the elements that allow it to be used as a

fertiliser or soil improver (European Commission, 2015).

2.7.1.Phosphorus

With the constant growing population the generation of human wastes is

alsogrowing up. Waste water is considered one of the main wastes

generated from humans and it is known that it is one of the main

contaminants of the environment because of its hazardousness (de-Bashan

and Bashan, 2004). Due to the large amounts of organic compounds and

other minerals many process have been developed to treat the wastewater.

However the companies have the need to invest in better process that can

be profitable, reliable and comply with the limits imposed by the

governments.

One of the most studied processes is related to phosphorus because

phosphorus treatment has the potential to be a profitable process.

Phosphorus is one of the most abundant elements on Earth. It is estimated

that there are 7000 million tonnes of phosphate rocks as P2O5 remaining in

reserves that could be economically mined. According to Shu et al. (2006),

the human population consumes 40 million tons of P as P2O5 each year and

it is predicted that P demand will increase by 1.5% each year.

Phosphorus is essential for all living organisms including humans who

depend on phosphorous to lead healthy and productive lives and as an

essential nutrient for crop production. Phosphorus represents the energy

currency for organisms at cell level, and its availability often controls

biological productivity; for that reason, in excess quantities, it is the cause of

eutrophication (Le Corre et al., 2009, Shu et al., 2006). Eutrophication is the

enrichment of nutrients of surfaces waters or other media, leading to

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excessive production of microbial algae resulting in toxic threat for the animal

life and is the responsible for turning water green in water bodies in general.

Human products such as fertilizers, detergents and insecticides contain a lot

of amounts of P as phosphates. Essentially the overdose of P in water

bodies in European Union (EU) countries comes from human sources in

sewage and from livestock (Morse et al., 1993). For that reason the

European legislation has regulated the maximun P concentration in effluents

depending on the size of discharge (EC Urban Waste Water Treatment

Directive 91/271/EEC, UWWTD, 1991).

Nowadays traditional processes of P removal (biological and chemical),

based on phosphorus fixation, are not enough, because they are efficient in

the sense that they can reduce the P concentration in wastewater effluents to

less than 1mg/L but they lead to the accumulation of phosphorus in the

sludge which represents one of the main problems for the European Union,

for the sludge disposal (Le Corre et al., 2009, Shu et al., 2006).

The pressure of fu